The present invention provides a mechanically self-stabilized cavity length controller for use within a ring laser gyroscope, including piezoelectric elements which are employed to move mirrors and effect a change of the laser path length. More particularly, a cavity length controller consisting of component parts fabricated from dissimilar materials, and configured so as to largely cancel any thermally-induced dimensional variations by virtue of the differential thermal expansion rates of the various component parts. The cavity length controller also includes two piezoelectric plates affixed to opposite sides of a deformable diaphragm which is coupled to a mirrored surface within the cavity. To correct for any nominal, thermally-induced cavity variations which might still be evident in a cavity employing the controller, an electrical potential is applied to the piezoelectric plates, causing the radial expansion of one plate and the simultaneous radial contraction of the other, and thereby deforming the diaphragm. This deformation results in a corresponding shift in the displacement of the coupled mirrored surface, effectively varying the length of the cavity which includes that mirrored surface.
Ring laser gyroscopes measure rates of rotation by detecting phase shifts in two counter-rotating beams of laser light within a cavity having a length equal to an integral number of wavelengths of the laser light being used. The cavity would include a lasing element, a light sensing element, and an arrangement of mirrors for directing the laser light into a closed path or "ring" within the cavity. The difference in travel time around the cavity for each of the beams (which is indicated by the number of phase shifts detected by the light sensing element) is directly proportional to the rotation of the cavity. This rotation-induced travel time difference is known as the Sagnac Effect. In order to insure that accurate measurements of rotation can be made with such a gyroscope, it is important that the distance which the beams travel around the cavity (the cavity length) be maintained constant to within about 0.001 of the wavelength of the laser light being used (a typical laser gyroscope employs a laser having a wavelength on the order of 600 nm).
The majority of modern ring laser gyroscope cavities are fashioned from ceramic glasses having extremely low coefficients of thermal expansion ("CTE"). Mirrors are placed within the cavity to direct the laser light into a closed path. While the use of low CTE glass minimizes the effect of temperature cycling upon the physical dimensions of the cavity, it does not eliminate it. It is therefore necessary to vary the positions of the mirrors within the cavity so as to compensate for the expansion and contraction of the cavity, and maintain a constant path length for the laser.
Cavity Length Controllers ("CLCs") are devices which alter the position of one or more of the mirrors contained within a ring laser gyroscope, and thereby introduce a controlled change in the cavity length. This cavity length adjustment has been effected in previous CLCs through the application of a direct current ("DC") voltage across a piezoelectric element. This element, which expands or contracts in response to applied voltages, would be coupled to cause the translation of a mirrored surface within the cavity and thereby produce a specific change in the cavity length. The applied voltage would then be adjusted to effect changes in the cavity length which would cancel thermally-induced cavity variations. In addition to the DC voltage applied across the piezoelectric element of a CLC, an alternating current ("AC") voltage is also typically applied. The AC voltage modulates the mirror controlled by the CLC about the position selected by the DC voltage. This positional modulation provides an error signal which is used to maintain the path length at a constant.
Ironically, one of the prime sources of thermally-induced cavity length variation has been the CLCs themselves. Prior art CLCs have typically contained components fabricated from materials having a higher CTE than the ceramic glasses which make up the remainder of the cavity. This increased length variation has made it necessary for the CLCs to compensate for greater distances, which for piezoelectric CLCs translates into a higher voltage being required for application to the piezoelectric element within them.
In a typical square laser cavity employing two prior art CLCs (such as ones having the configuration disclosed by U.S. Pat. No. 4,836,677), potentials in excess of 200 V would have to be applied to each of the CLCs in order to compensate for cavity length variations over a -55.degree. C. to +70.degree. C. temperature range. This type of high-voltage requirement has limited the temperature ranges over which laser gyroscopes employing prior art CLCs could reliably operate (as many applications of ring laser gyroscopes are in vehicles or weapon systems, available power is usually at a premium). In addition, since the amount of thermal expansion of a laser cavity is directly proportional to the size of the cavity, limitations which these high-voltage requirements place upon achievable mirror displacement have prohibited the application of piezoelectric CLCs in larger laser cavities.
Accordingly, it is the object of the present invention to provide for a piezoelectric CLC which exhibits high dimensional stability over a wide range of temperatures, thus reducing the need for large mirror displacements, and associated high-voltages that need be applied to the CLC in order to effect them. This will allow CLCs having the invention's configuration to maintain a constant path length within a laser cavity over a greater range of temperature for any given voltage restriction, and to be implemented in laser cavities of almost any size.